The bleaching of raw cotton and wood pulp are massive industries.
Raw cotton originating from cotton seeds contains mainly colourless cellulose, but has a yellow-brownish colour due to the natural pigment in the plant. Many impurities adhere, especially to the surface. They consist mainly of protein, pectin, ash and wax.
The cotton and textile industries recognise a need for bleaching cotton prior to its use in textiles and other areas. The cotton fibres are bleached to remove natural and adventitious impurities with the concurrent production of substantially whiter material.
There have been two major types of bleach used in the cotton industry. One type is a dilute alkali or alkaline earth metal hypochlorite solution. The most common types of such hypochlorite solutions are sodium hypochlorite and calcium hypochlorite. Additionally, chlorine dioxide as bleaching agent has been developed and shows less cotton damage than hypochlorite does. Also mixtures of chlorine dioxide and hypochlorite can be applied. The second type of bleach is a peroxide solution, e.g., hydrogen peroxide solutions. This bleaching process is typically applied at high temperatures, i.e. 80 to 100° C. Controlling the peroxide decomposition due to trace metals is key to successfully apply hydrogen peroxide. Often Mg-silicates or sequestering agents such as EDTA or analogous phosphonates can be applied to reduce decomposition.
The above types of bleaching solutions and caustic scouring solutions may cause tendering of the cotton fibre due to oxidation which occurs in the presence of hot alkali or from the uncontrolled action of hypochlorite solutions during the bleaching process. Also hydrogen peroxide is known to give reduced cotton fibre strengths, especially when applied without proper sequestration or stabilisation of transition-metal ions. Tendering can also occur during acid scours by the attack of the acid on the cotton fibre with the formation of hydrocellulose.
Purified cellulose for rayon production usually comes from specially processed wood pulp. It is sometimes referred to as “dissolving cellulose” or “dissolving pulp” to distinguish it from lower grade pulps used for papermaking and other purposes. Dissolving cellulose is characterised by a high cellulose content, i.e., it is composed of long-chain molecules, relatively free from lignin and hemicelluloses, or other short-chain carbohydrates. A manufactured fibre composed of regenerated cellulose, in which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups.
Wood pulp produced for paper manufacture either contains most of the originally present lignin and is then called mechanical pulp or it has been chiefly delignified, as in chemical pulp. Different sources of wood pulp can be found, such as softwood pulp (from e.g., fir trees), or hardwood pulp, such as that originating from birch or eucalyptus trees. Mechanical pulp is used for e.g. newsprint and is often more yellow than paper produced from chemical pulp (such as for copy paper or book-print paper). Further, paper produced from mechanical pulp is prone to yellowing due to light- or temperature-induced oxidation. Whilst for mechanical pulp production mild bleaching processes are applied, to produce chemical pulp having a high whiteness, various bleaching and delignification processes are applied. Widely applied bleaches include elemental chlorine, chlorine dioxide, hydrogen peroxide, and ozone.
Whilst for both textile bleaching and wood pulp bleaching, chlorine-based bleaches are often most effective, there is a need to apply oxygen-based bleaches for environmental reasons. Hydrogen peroxide is a good bleaching agent; however, it needs to be applied at high temperatures and long reaction times. For industry it is desirable to be able to apply hydrogen peroxide at lower temperatures and shorter reaction times than in current processes.
The macrocyclic triazacyclic molecules have been known for several decades, and their complexation chemistry with a large variety of metal ions has been studied thoroughly. The azacyclic molecules often lead to complexes with enhanced thermodynamic and kinetic stability with respect to metal ion dissociation, compared to their open-chain analogues.
EP 0458397 discloses the use manganese 1,4,7-Trimethyl-1,4,7-triazacyclononane (Me3-TACN) complexes as bleaching and oxidation catalysts and use for paper/pulp bleaching and textile bleaching processes. 1,4,7-Trimethyl-1,4,7-triazacyclononane (Me3-TACN) has been used in dishwashing for automatic dishwashers, SUN™, and has also been used in a laundry detergent composition, OMO Power™. The ligand (Me3-TACN) is used in the form of its manganese transition metal complex, the complex having a counter ion that prevents deliquescence of the complex.
United States Application 2001/0025695A1, Patt et al, discloses the use of PF6− salts of 1,2,-bis-(4,7,-dimethyl-1,4,7,-triazacyclonon-1-yl)-ethane and Me3-TACN (Me4-DTNE).
United States Application 2002/010120 discloses the bleaching of substrates in an aqueous medium, the aqueous medium comprising a transition metal catalyst and hydrogen peroxide.
WO 2006/125517 discloses a method of catalytically treating a cellulose or starch substrate with a Mn(III) or Mn(IV) preformed transition metal catalyst salt and hydrogen peroxide in an aqueous solution. The preformed transition metal catalyst salt is described as having a non-coordinating counter ion and having a water solubility of at least 30 g/l at 20° C. Exemplified ligands of the catalysts described in WO 2006/125517 are 1,4,7-Trimethyl-1,4,7-triazacyclononane (Me3-TACN) and 1,2,-bis-(4,7,-dimethyl-1,4,7,-triazacyclonon-1-yl)-ethane (Me4-DTNE).